Many utility providers are now deploying “smart grid” technologies into respective portions of the power grid. Smart grid technologies may enhance the respective abilities of utility providers and their consumers to communicate with each other and to decide how and when electrical energy should be produced or consumed.
Some smart grid applications may enable time-based electricity pricing. For example, a consumer who typically pays a fixed rate per kilowatt-hour (kWh) may configure an appliance equipped with a smart grid application, e.g., to set an electricity usage threshold for the appliance or to adjust the appliance's electricity usage behavior, to take advantage of fluctuating prices. Additionally, smart grid applications may enhance opportunities for demand response, e.g., by providing real time power consumption data to electricity producers or consumers. An example of providing real time power consumption data includes measuring the power consumption of a load at one or more respective points in time over a given interval, averaging the power consumption values measured during the interval, and providing the averaged power consumption as the real time power consumption of the load.
Smart grid applications may require the use of an energy management system (EMS) to provide automated control and/or monitoring of electricity-consuming devices, such as lighting, appliances, etc. An EMS may collect electricity usage data, such as power consumption data, from the devices, and may use that data to perform self-diagnostic and optimization routines, to perform trend analysis, to generate annual consumption forecasts, and the like.
Load control devices, such as electronic switches and dimmers, may be used to control the amount of power delivered from an alternating current (AC) source to a load that consumes the power. Such a load control device can be configured to calculate an amount of power consumed by a load. The power consumption data may be communicated from the load control device to an EMS or a user of the load control device.
FIG. 1A depicts an example load control system 100 that includes a three-wire load control device 104. The three-wire load control device 104 may be electrically connected between an AC source 108 and a load 110. The three-wire load control device 104 may be operable to control an amount of power delivered from the AC source 108 to the load 110. The load 110 may be a lighting load, for example, or any other electrical load.
The three-wire load control device 104 may be an electronic switch or dimmer for example. The three-wire load control device 104 may include a controllably conductive device, such as a thyristor (e.g., a triac), operable to control an amount of power delivered from the AC source 108 to the load 110. The three-wire load control device 104 may be connected to the AC source 108 by a first wire 112, to the load 110 by a second wire 114, and to an electrical path between the load 110 and a return side of the AC source 108 by a third wire 116. The first wire 112 may be referred to as a hot wire, the second wire 114 may be referred to as a switched-hot or dimmed-hot wire, and the third wire 116 may be referred to as a neutral wire.
The controllably conductive device (not shown) may operate in respective non-conductive and conductive states within respective portions of each half-cycle of an AC waveform provided by the AC source. The controllably conductive device may be switched from the non-conductive to the conductive state in response to a triggering signal. In a forward phase control system, generation of a triggering signal may be synchronized with the AC line voltage such that the triggering signal is generated at a certain time after a zero crossing is detected. Responsive to the triggering signal, a gate of the controllably conductive device may be energized, causing the controllably conductive device to operate in the conductive state for the remainder of the AC half-cycle.
During the time interval between the zero crossing and the gate triggering, the controllably conductive device may operate in the non-conductive state. When the controllably conductive device is operating in the non-conductive state, effectively no power is supplied to the load. The load control device may be configured to allow for alteration of the time interval, such as in response to adjustment of a user-operable control (e.g., a dimming knob or a slider) or in response to changes in a dimming level signal. Altering the time interval between the zero crossing and the gate triggering (and, thereby affecting the conduction angle of the controllably conductive device) affects the root-mean-square (RMS) power delivered to the load 110. See, for example, commonly-assigned U.S. Pat. No. 5,430,356, entitled “Programmable Lighting Control System With Normalized Dimming For Different Light Sources,” the entire disclosure of which is incorporated herein by reference. Thus, the controllably conductive device may be switched to affect the AC voltage waveform provided to the load 110, thereby controlling the power delivered to the load.
The three-wire load control device 104 has three AC waveforms available to it: an AC input voltage waveform 118, which may be measured between the first wire 112 and the third wire 116; an AC device voltage waveform 120, which may be measured between the first wire 112 and the second wire 114; and an AC load voltage waveform 122, which may be measured between the second wire 114 and the third wire 116.
Because the three-wire load control device 104 always has the AC load voltage waveform 122 available to it, the three-wire load control device 104 can be configured to compute power consumed by the load 110 over a given time interval. Power consumed by the load 110 may be determined by multiplying the voltage drop across the load 110 by the current flowing through the load 110. The three-wire load control device 104 can be configured to measure the current it supplies to the load 110 during any given time interval. And, because the three-wire load control device 104 has the AC load voltage waveform available 122 to it, it can also be configured to determine the voltage drop across the load 110 during any given time interval. Thus, because the three-wire load control device 104 always has available to it both the current supplied to the load 110 and voltage dropped across the load 110, the three-wire load control device 104 can be configured to compute power consumed by the load 110 over any given time interval.
FIG. 1B depicts an example load control system 102 that includes a two-wire load control device 106. The two-wire load control device 106 may be electrically connected between an AC power source 108 and a load 110. The two-wire load control device 106 may be operable to control an amount of delivered from the AC source 108 to the load 110.
The two-wire load control device 106 may be an electronic switch or dimmer for example. The two-wire load control device 106 may include a controllably conductive device, such as a thyristor (e.g., a triac), operable to control an amount of power delivered from the AC power source 108 to the load 110. The two-wire load control device 106 may be connected to the AC power source 108 by a first wire 112 and to the load 110 by a second wire 114. The first wire 112 may be referred to as a hot wire. The second wire 114 may be referred to as a switched-hot or dimmed-hot wire. As described above, the controllably conductive device (not shown) may operate in respective non-conductive and conductive states within respective portions of each half-cycle of an AC input voltage waveform provided by the AC power source.
Unlike the three-wire load control device 104 described above, the two-wire load control device 106 does not have the AC load voltage waveform 122 available to it. Accordingly, the two-wire load control device 106 cannot be configured to determine the actual voltage drop across the load 110. Consequently, the two-wire load control device 106 cannot be configured to compute the actual power consumed by the load 110 over any given time interval by merely multiplying the actual voltage drop across the load by the actual current flowing through the load during the given time interval. It would be desirable if there were available a two-wire load control device capable of computing and reporting an accurate estimate of real-time power consumption of a corresponding load.